Electrodeposition is generally a plating process that uses electrical current to reduce or oxidize chemical species of a desired material from a solution and coat a conductive substrate with a thin layer of that material. An electroplating system typically includes two electrodes and an electrolyte. Additionally, a reference electrode may also sometimes be employed. In an electrodeposition process, typically the part to be coated is one of the electrodes and the coating material is supplied from the electrolyte in which the electrodes are immersed. In electroplating, the electrolyte is replenished periodically with the chemical species being deposited on the substrate. Sometimes, the electrode that is not being coated can be a source of the chemical species in order to replenish the electrolytic solution.
Solar or photovoltaic cells are devices that convert photons into electricity by the photovoltaic effect. Solar cells are assembled together to make solar panels, solar modules, or photovoltaic arrays. Thin film solar cells are stacked structures, having layers of materials, including photovoltaic materials, stacked on a substrate for support of the stack. There are many fabrication techniques used for fabricating the individual layers of the stack. One particularly useful method is electrodeposition, however there are drawbacks to conventional apparatus and methods in this respect. For example, when electrodepositing a material onto an electrically insulating substrate, such as glass, a conductive coating must be applied to the substrate in order to allow electric currents to pass. These conductive coatings are typically thin and can have high sheet resistance which produces a voltage drop and current non-uniformities when electroplating over a large area. In these cases uniform deposition of the electroplated film is problematic.
What is needed, therefore, are improved apparatus and methods for electrodeposition on large area, resistive substrates. Given the demand for renewable energy, improved apparatus and methods are particularly important for solar cell fabrication where the typical substrate is glass coated by a thin layer of transparent conductive oxide.